Help with an alpha brush - camera setup
your sample bias setting is screwing you over here. I tried your first file, and just loading as a texture, and using it without any editing, on Area plane, anchored mode, and custom falloff set to all max, it worked fine. I would however suggest you didn't use the fade on the edges, seeing how this alpha is a full object with hard edges, and not truly just a texture, but a belt buckle...?
in any case, blender accepts black-backed alphas by default, because they're easier to produce and provide much more detail in the most often used direction. Generally, grey-backed alphas (the background grey at 50% brightness) is used in alphas with both positive and negative deformation.